1. Field of the Invention
This invention relates to electronic article surveillance (EAS) systems for detecting the unauthorized removal of objects from a protected area, and in particular, to such systems in which an alternating magnetic field is produced in an interrogation zone thereby enabling the detection of a ferromagnetic marker.
2. Description of the Prior Art
EAS systems based on the detection of a ferromagnetic marker are now well known, having been disclosed at least as early as 1934 in French patent No. 763,681 (Picard) and more recently disclosed in patents as U.S. Pat. Nos. 3,665,449, 4,135,183 and the like. In all such systems, an interrogation antenna usually comprising at least one coil several feet wide, is positioned within a pedestal on one side of an exit way and is energized to generate a magnetic field alternating at a predetermined frequency. The field encompasses a zone within which an object carried by a person passing through the zone would encounter the produced field. In such a system, a detector coil is typically positioned within the same pedestal or may be located within a second pedestal spaced from the first pedestal to define the interrogation zone therebetween, and the detector coil is connected to a signal detector and alarm indicator circuit. When an object, the unauthorized removal of which is to be prevented, is provided with a ferromagnetic marker, the interaction of the alternating field with the marker creates harmonics of the predetermined frequency, and those harmonics are picked up by the detector coil and processed to indicate the presence of a marker. The magnetic fields produced within the interrogation zone may also induce currents in other conductive and/or magnetic objects which may be proximate to the interrogation zone. Such induced currents and/or magnetically produced responses may result in the production of false alarms and/or artificially high electromagnetic background signals.